The present invention relates to a substantially water-impermeable polyolefin foam made of expandable beads by bead foaming and a process for preparing the same.
In recent years, a polyolefin foam made by bead foaming has been employed widely in uses such as a vessel, a float, a buoy and a heat insulating material. However, a conventional polyolefin foam made by bead foaming has the disadvantage that when the foam keeps contacting with a liquid such as water, the liquid permeates into and is included in the foam in a short period of time, consequently resulting in various troubles such as leakage of liquid from a vessel made of the foam, decrease of buoyancy and decrease of heat insulating effect. For such a reason, there are strongly desired a water-impermeable polyolefin foam and a bead foaming process for the industrial preparation thereof.
The foaming technique concerning the polyolefin foam by bead foaming is attended with high difficulties, and its technical development was behind the development of polystyrene foam. Polystyrene is in general an amorphous high polymer and, therefore, the bead foaming thereof is carried out by utilizing the change in characteristics in the vicinity of the glass transition temperature and is relatively easy, whereas a polyolefin is in general a crystalline high polymer and, therefore, the bead foaming thereof is carried out by utilizing the change in characteristics in the vicinity of the melting point of crystal region. However, the control of the foaming condition of a polyolefin is very difficult because the change in characteristics in the vicinity of the melting point of crystal region is very sudden as compared with the change in the vicinity of the glass transition temperature. Consequentially, in case of a polyolefin foam made by bead foaming, the condition of the interface between the melt-adhered beads is apt to become bad, and after foaming, a large number of not melt-adhered portions are apt to remain between beads as voids or gaps.
It is an object of the present invention to provide a novel polyolefin foam which is substantially impervious to water.
A further object of the present invention is to provide a process for preparing a substantially water-impermeable polyolefin foam by bead foaming.
These and other objects of the present invention will become apparent from the description hereinafter.